1. Field of the Invention
This invention relates to synchronizing, or sync, signal extracting apparatus, such as the apparatus to extract the block sync signals which are used in digital video tape recorders (hereinafter abbreviated as DVTR).
2. Description of the Prior Art
In digital data transmission, data information is usually transmitted together with synchronizing information so that the decoding device which is to receive such data information can properly break that information into the symbolic or pictorial elements of which it is comprised.
In digital data transmission bit synchronization, word synchronization and frame synchronization are all used. Of these forms of synchronization, bit synchronization is not directly related to the present invention and hence discussion of it will be omitted herein. The present invention, however, can be used to achieve either word or frame synchronization, but word synchronization can easily be obtained by counting bits within a given frame once frame synchronization is obtained, and hence does not require further discussion, except to state that when 1 frame is composed of 1 word, synchronization and frame synchronization are the same. This form of synchronization, where one word equals one frame will hereinafter be called block synchronization.
Ideally apparatus for decoding a data transmission that uses block synchronization should be able to quickly recover the timing of the block synchronization whenever such timing is lost, that is it should have good recovery characteristics and it should be able to maintain such timing once obtained in the presence of noise, that is it should have good holding characteristics. Unfortunately these two characteristics are contradictory. In order to improve the probability of accurately detecting the block sync signals, the length of the synchronous bit pattern used to indicate block synchronization may be increased so as to decrease the likelihood that such a synchronous bit pattern will occur as part of the data portion of a digital transmission. But, the use of such lengthened synchronous bit patterns results in a degradation of information transmitting efficiency. As a result of these trade offs, an optimum synchronizing system must be tailored for each of the various kinds of transmission systems.
A variety of systems have been proposed for recovering synchronization timing once such timing has been lost, such as the 1 bit shift system, the reset. system, and the block correlating system.
The 1 bit shift system attempts to correct for mis-synchronization by shifting its block synchronization one bit at a time. As a result, it is a its average recovery time is relatively long. This 1 bit shift system is effective for recovering from the slip-off of synchronization of a small number of bits such as might result from clock slip or the like, but it requires the provision of a phase comparator for the purpose of recognizing the direction of the clock slip.
In the reset system, if the synchronous bit pattern is detected at an intermediary portion of the block the system resynchronizes to such synchronous bit pattern.
In the above mentioned 1 bit shift system and the reset system, if the same pattern as the synchronous bit pattern appears in the information sequence, mis-synchronizaton can occur. Such mis-synchronization can be prevented by comparing the timing of all of the block pulses in the course of recovering the correction synchronization. Such a system is the above mentioned block correlating system. This block correlating system is quite effective in detecting the synchronization of information signals being supplied continuously in real time.
In many types of data transmissions, such as in many types of data transmissions between computers, if uncertainty regarding the synchronization in a given portion of data is detected, it is possible to retransmit that portion until it is received with proper synchronization. But in an apparatus for decoding in real time continuously arriving information, such as the video and audio signals handled by a DVTR, data which is not properly synchronized is lost. As a result, such real time decoding apparatus must be provided with synchronization information even if there is doubt about the reliability of that synchronization information because of the possibility of mis-synchronization of drop-out. Furthermore, in DVTRs using a rotary head, the transducers of the DVTRs are transferred from one track to another on the recording tape in rapid succession. As a result, it is necessary to rapidly establish synchronizaton timing on each such track that is scanned by the transducers.
Apparatus for decoding in real time continuously arriving information, such as a DVTR, must have a level of reliability similar to that required for usual data transmission and at the same time have a capability of quickly responding to an error in synchronization when it occurs and of quickly re-establishing proper synchronization.
DVTRs in particular require the ability to quickly detect synchronization errors and to rapidly re-establish proper synchronization, because they reproduce digital video signals from tape. Tape noise, the drop out of synchronous signals and tape jitter all increase the probability synchronization errors. Therefore, without the ability to rapidly correct for such synchronization errors, a large percent of the video information read by such a DVTR would be improperly reproduced.